1. Field of the Invention
This invention relates to a piston for an internal combustion engine, primarily a diesel engine. The piston is of the type having a cavity for a combustion chamber near the center of the top surface thereof.
2. Description of the Prior Art
FIG. 1 illustrates a prior art piston of the type disclosed in Japanese Unexamined Publication Sho 58-72654, for example, which has a cavity forming member b provided with a cavity a at a central area of the top surface thereof. The member b is a separate element from a piston main body c, and is supported on a piston pin d provided laterally in the piston body c. The cavity forming member b is made of a low thermal conductive material such as cast iron or the like, and the piston main body c is made of a small specific gravity material such as an aluminum alloy or the like so that the overall weight of the piston is small. This has the advantage that the engine can be operated at a high speed and the weight can be decreased.
A piston of this type, however, is usually constructed so that a top surface of the cavity forming member b has an enlarged flange-shaped portion e having the same diameter as the piston main body c, and the cavity forming member b with the enlarged flange-shaped portion e is placed on the top surface of the piston main body c, as shown in FIG. 1. In this arrangement the cavity forming member b is mounted on the piston main body c with a small clearance f formed therebetween for allowing for movement due to a difference in the thermal expansion coefficient between the member b and the piston main body c. As a result, the cavity forming member b often has a swivel motion corresponding to the extent of the clearance f about the piston pin d. There is a possibility of a scuffing or biting between the flange-shaped portion e and a cylinder as a result of the swivel motion.